set ASYNC_LOW_LATENCY flag on serial device just like setserial does to fix huge latency found on recent kernel versions

CMakeLists.txt

@@ -53,6 +53,7 @@ set(CMAKE_REQUIRED_LIBRARIES pthread rt)
 check_function_exists(pthread_setname_np HAVE_PTHREAD_SETNAME_NP)
 check_function_exists(pselect HAVE_PSELECT)
 check_function_exists(ppoll HAVE_PPOLL)
+check_include_file(linux/serial.h HAVE_LINUX_SERIAL -DHAVE_LINUX_SERIAL=1)
 check_function_exists(argp_parse HAVE_ARGP)
 if(NOT HAVE_ARGP)
   find_library(LIB_ARGP argp)

config.h.cmake

@@ -13,6 +13,9 @@
 /* Defined if pselect() is available. */
 #cmakedefine HAVE_PSELECT
 
+/* Defined if linux/serial.h is available. */
+#cmakedefine HAVE_LINUX_SERIAL
+
 /* Defined if pthread_setname_np is available. */
 #cmakedefine HAVE_PTHREAD_SETNAME_NP
 

configure.ac

@@ -35,6 +35,7 @@ AC_SUBST(EXTRA_LIBS)
 
 AC_CHECK_FUNC([pselect], [AC_DEFINE(HAVE_PSELECT, [1], [Defined if pselect() is available.])])
 AC_CHECK_FUNC([ppoll], [AC_DEFINE(HAVE_PPOLL, [1], [Defined if ppoll() is available.])])
+AC_CHECK_HEADER([linux/serial.h], [AC_DEFINE(HAVE_LINUX_SERIAL, [1], [Defined if linux/serial.h is available.])])
 
 AC_ARG_ENABLE(coverage, AS_HELP_STRING([--enable-coverage], [enable code coverage tracking]), [CXXFLAGS+=" -coverage -O0"], [])
 AC_ARG_WITH(contrib, AS_HELP_STRING([--without-contrib], [disable inclusion of contributed sources]), [], [with_contrib=yes])

src/lib/ebus/device.cpp

@@ -27,6 +27,9 @@
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <netinet/tcp.h>
+#ifdef HAVE_LINUX_SERIAL
+#  include <linux/serial.h>
+#endif
 #include <errno.h>
 #ifdef HAVE_PPOLL
 #  include <poll.h>
@@ -190,6 +193,14 @@ result_t SerialDevice::open() {
     return RESULT_ERR_DEVICE;
   }
 
+#ifdef HAVE_LINUX_SERIAL
+  struct serial_struct serial;
+  if (ioctl(m_fd, TIOCGSERIAL, &serial) == 0) {
+    serial.flags |= ASYNC_LOW_LATENCY;
+    ioctl(m_fd, TIOCSSERIAL, &serial);
+  }
+#endif
+
   // save current settings
   tcgetattr(m_fd, &m_oldSettings);